1. Field of the Invention
The invention relates to pipes, tubing and containers having a wall structure composed of fiber reinforced polymer composites.
2. Description of Related Art
Fiber reinforced plastic (FRP) composites are finding increased usage as piping in chemical plants as well as casing used in the drilling of oil and gas wells and casing and tubing for the transport of crude oil and natural gas up from the well source. These materials are also useful in the construction of containers such as pressure vessels and underground or above ground storage tanks.
The advantage of FRP composites over carbon steel structures in oil/gas applications includes superior corrosion resistance, flexibility in achieving mechanical property design targets and improved fatigue resistance. FRP composites are also of considerably lighter weight for a given wall thickness than their steel counterparts.
FRP structures designed for use in high pressure piping or casing such as crude oil pipelines and oil well tubing are generally prepared by impregnating a roving of filaments of a high strength material, such as continuous glass filaments, with a thermosettable resin composition, such as an epoxy resin, and winding the impregnated filaments back and forth onto a mandrel under tension to form a plurality of intermeshed filament windings. Filaments may be wound at an angle of 90.degree. to the pipe axis or at angles of 0.degree. to .ltoreq.+/-90.degree., e.g., .+-.88.degree.) with respect to the pipe axis, in which latter case a helical filament winding pattern is formed. After a desired pipe wall thickness is achieved, the winding operation is discontinued, the resin is cured and the mandrel is extracted resting in a cylindrical pipe having a fiber reinforced wall structure. FRP pipes of this type and their method of production are disclosed, e.g., in U.S. Pat. Nos. 2,843,153 and 5,330,807, the complete disclosures of which patents are incorporated herein by reference. Larger diameter structures such as containers may be fabricated the same way using larger diameter mandrels.
FRP pipe designed for use in onshore or onshore fossil fuel recovery must be constructed to withstand two basic forces to which it will be subjected. The first force is an outer radial load exerted along a vector normal to the pipe walls by fluids (oil or drilling muds) which are conveyed under moderate to high pressure through the pipe, also known as the hoop load. The second force is an axial tensile load exerted along vectors parallel to the pipe axis and occasioned by the fluid pressure and/or by the weight of a long string of coupled pipe sections for applications where they are suspended, in the ground at the well bore and/or between the well bore and surface platform in offshore recovery operations. These strings are often suspended 3,000 to 10,000 feet (about 850 to 2800 meters), and thus must be able to carry a long term axial stress in excess of about 2500 pounds per square inch (or 2.5 ksi) occasioned during operation and when the pipe sting is inserted and removed during the fossil fuel recovery process. Other structures such as storage tanks and pressure vessels are designed primarily to maximize containment capability in a direction normal to the tank or vessel longitudinal axis, i.e., hoop load.
FRP pipe having maximum hoop strength can be designed if the reinforcing fiber is wound at an angle close to 90.degree. to the pipe axis, e.g., +/-70.degree. up to 90.degree.. Conversely, maximum tensile strength is developed where the reinforcing fiber is applied at an angle close to 0.degree. to the pipe axis, e.g. +/-30.degree. down to 0.degree.. However, pipe wound at or close to 90.degree. exhibits sever diminishment of axial tensile strength while pipe wound at or close to 0.degree. exhibits severe diminishment of hoop strength. Pipe wound at intermediate pipe axis angles between +/-30.degree. to +/-70.degree. (as disclosed in U.S. Pat. No. 2,843,153) generally compromises hoop and particularly axial strength.
One technique for attempting to maximize both hoop and axial strength in pipe manufacture is to lay down the reinforcing fiber composite as separate laminate layers one atop another, each layer having the fibers disposed at different pipe axial angles designed to maximize the hoop or axial stress bearing properties of the pipe as well as minimize the coefficient of expansion of the composite pipe. An example of such a construction containing +/-20.degree. to +/-60.degree. fiber layers alternating with 90.degree. layers is disclosed in U.S. Pat. No. 5,330,807. Other similar layered laminates are disclosed in U.S. Pat. Nos. 4,728,224 and 4,385,644.
FRP composites currently commercially available also may exhibit a serious deficiency which makes their use not cost effective in applications that generate even moderate containment stress. For example, microcracking and delamination of pipe wall structure at or near the pipe joints and/or along the pipe length provide a leak path for fluids, commonly referred to as "weeping", which can occur at fluid pressures which can be 5 to 10 times.ltoreq.the pipe short-term burst pressure. Intrusion of water into the pipe wall structure via these microcracks can attack glass fiber surfaces and/or binder resin, leading to ply delamination of composite laminated structures and pure pipe failure. Other devices such as FRP underground storage tanks also exhibit premature microcracking and thus their corrosion resistance does not offer any additional advantages over similar metal structure.
Although microcracking can be mitigated by increasing the structural wall thickness, this solution drives the composite pipe cost up as compared to that of carbon steel structures. The higher cost constitutes a barrier to the substitution of FRP composite pipes for carbon steel in moderate to high (injection) pressure applications. Also, in downhole applications, the increased wall thickness prevents the use of composites where the diameter of the well bore is constrained, because of the cross-sectional area available for fluids to flow is smaller than that for carbon steel. The use of composites in these applications would require drill holes with larger diameter, and this gives rise to additional drilling costs.
Accordingly, it is a primary object of this invention to provide composite FRP structures having improved containment strength and which are more resistant to microcracking and delamination on the one hand and also have diminished wall thickness on the other hand such that the structure, e.g. pipe, is more compatible with carbon steel well bore/casing dimensions.